The genome of HT-1 MSV is modified by recombinant DNA techniques to contain a bacterial origin of DNA replication and a selectable drug-resistance gene. The covalently closed circular proviral DNA intermediate of this chimeric retro-virus is capable of replicating in bacteria as a plasmid. This novel retro-virus vector will be used 1) to introduce foreign DNA into mouse 3T3 cells under retroviral control and 2) to study effect of mutations on the process of reverse-transcription and integration.